Application Cases

Experiment Name: Research on Direct Detection Technology of Space Charge in Polyimide under Square-Wave Pulsed Electric Fields

Research Direction: Polyimide films are widely used in the insulation systems of variable-frequency speed-regulating motors due to their excellent electrical properties. However, the rapid polarity reversal process of square-wave electric fields (at the ns-μs level) poses challenges for directly detecting space charge inside the dielectric. This study employs the pulsed electroacoustic (PEA) method, optimizing system parameters to achieve direct dynamic detection of space charge under square-wave electric fields, with a phase resolution of 1.79° and reduced voltage waveform distortion. The research found that untreated polyimide exhibits bipolar carrier injection and deep trap injection under DC electric fields, leading to long-term accumulation of space charge. In contrast, under square-wave electric fields, space charge primarily originates from homopolar charge injection from the lower aluminum electrode and can be rapidly extracted during voltage polarity reversal, with no significant accumulation observed. Heat treatment may disrupt polyimide molecular chains, affecting surface morphology and increasing high-frequency dielectric constants, but it has no significant impact on breakdown resistance. Heat treatment reduces positive charge injection under DC electric fields and accelerates space charge dissipation but has limited influence on space charge distribution under square-wave electric fields. Additionally, rise times in the range of 100 μs to 1000 μs have minimal effect on the accumulation and distribution of space charge.

Experiment Objective: Direct detection of space charge in polyimide under square-wave pulsed electric fields.

Test Equipment: Function generator, high-voltage amplifier, high-voltage high-frequency pulse source, voltage divider, digital oscilloscope, computer.

Experimental Process:
When the output signal is set to a square-wave waveform, the frequency range is from 1 μHz to 50 MHz, with rise and fall times not exceeding 5 ns. The dual-channel function generator outputs an adjustable periodic voltage signal in frequency and amplitude through one channel, which is applied to the test sample after being amplified by the high-voltage amplifier. The high-voltage amplifier has a maximum output voltage of 20 kV. The other channel of the function generator outputs a periodic TTL timing signal, simultaneously controlling the high-voltage high-frequency pulse source to generate pulse excitation and triggering the digital oscilloscope for data acquisition. The high-voltage high-frequency pulse source, also a self-developed laboratory device, is set to trigger on the rising edge of the TTL signal, outputting a high-frequency pulse excitation with an adjustable pulse width (5 ns–20 ns) each time. The pulse amplitude can reach up to 2.5 kV, with a maximum repetition frequency of 3 kHz. The high-voltage pulse is coupled to the test sample via a protective capacitor, the primary function of which is to prevent the high-voltage amplifier output from damaging the pulse source. The oscilloscope, equipped with large-capacity data storage, is also triggered on the TTL rising edge, ensuring that each space charge signal generated by the pulse excitation is recorded. The PEA electrode system is a self-developed and assembled setup, with the upper electrode made of semiconductive material and the lower electrode made of aluminum. The PVDF piezoelectric sensor converts the acoustic signals generated by charge-induced vibrations into electrical signals, which are transmitted to the oscilloscope via an amplifier for reading and storage. The oscilloscope simultaneously records the voltage applied to the sample at each moment for subsequent data processing.

Figure 1-1: Experimental Flow Block Diagram

Experimental Results:
In this study, the square-wave electric field frequency $f_a=50\text{Hz}$. Based on the characteristic equation of the AEPS principle, the pulse excitation frequency is set to $f_p=2010\text{Hz}$. The minimum number of square-wave cycles $N_a$ for repeated detection phases is 5, during which the number of pulses $N_p$ is 201. According to the phase resolution formula (2-4), the phase resolution ${\Delta }^{\prime }$ is approximately 1.79°. The figure better illustrates the measurement principle of AEPS. With a square-wave frequency of 50 Hz and a pulse excitation frequency of 2010 Hz, each square-wave cycle contains $2010/50=40.2$ pulse excitations, meaning 40 non-repeating pulse excitations per square-wave cycle. Assuming the first pulse at the start of the first square-wave cycle is at a phase of 0°, the 41st pulse corresponds to a square-wave phase of $41/2010\times 50\times 360=367.16°$, or 7.16°. Pulses 41 to 80 correspond to the second square-wave cycle, with phases distinct from those in the first cycle. Similarly, the 81st pulse corresponds to a phase of 5.37°, the 121st pulse to 3.58°, and the 161st pulse to 1.79°.

Figure 1-2: Schematic Diagram of AEPS Acquisition under Square-Wave Electric Fields

Product Recommendation: ATA-7100 High-Voltage Amplifier

Figure: ATA-7100 High-Voltage Amplifier Specifications

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